It is a mainstream that the voice coil motor is of the movable coil type in which the stator includes a permanent magnet, which becomes a magnetic field, and the movable element includes an armature coil.
Concerning the stator, the following two types are used.
(1) When a view is taken from the side, the stator is formed into an E-shape, and one end of the yoke is open, so that the movable element can be detached.
(2) When a view is taken from the side, the stator is formed into a θ-shape, and one end of the yoke is closed, so that the movable element can not be detached.
The movable element includes an air-core coil formed into a cylindrical shape or a square cylindrical shape. Alternatively, the movable element includes a coil wound round a non-magnetic thin bobbin made of engineering plastics or light metal.
FIG. 4 is a projection drawing showing a voice coil motor of the prior art, the side of the stator of which is formed into the θ-shape, wherein FIG. 4(a) is a partially sectional front view, and FIG. 4(b) is a side view.
In FIG. 4, the stator 41 includes: a yoke 42 made of iron; a permanent magnet 43; and a center yoke 44. In many cases, the thin copper sheet 46 is wound round the center yoke 44 so as to reduce inductance.
The movable element 47 includes: a coil 48; and a movable element attaching member 49. In some cases, the movable element 47 includes a bobbin 52 (shown in FIG. 5) round which the coil 48 is wound.
The movable element 47 is held by a bearing not shown capable of moving linearly in the axial direction. When a DC current is made to flow in the coil 48, the movable element 47 can be moved in the arrowed direction (shown in FIG. 4(a)) by the Fleming's left-hand rule. When the polarity of the electric current is inverted, the moving direction can be reversed.
However, the following problems are caused in the above voice coil motor of the prior art.
FIG. 5 is a perspective view of the movable element of the conventional voice coil motor. In FIG. 5, the thickness t of the coil 48 in the wire stacking direction greatly affects the magnetic circuit of the stator. Therefore, it is impossible to extremely reduce the thickness t of the coil 48. Accordingly, the rigidity of the movable element relies on only the non-magnetic thin bobbin 52 or on varnish and impregnation resin applied at the time of winding the coil. Therefore, the rigidity of the movable element is relatively low. This relatively low rigidity hinders improvements in the control characteristic of the voice coil motor.